Most chemical compounds exhibit
characteristic infrared spectra, depending on their molecular symmetry, atomic
weights and bond strength. This is an advantage when IR is used for unknown
compounds identification, but could be a disadvantage when seeking window
materials, which are desired not to have infrared spectrum. Such materials are
rare and have several disadvantages. Traditional window materials used in IR
spectroscopy are hygroscopic (KBr, NaCl, CsI), fragile, chemically not
resistant, expensive (diamond), or are totally absorbing in a part of infrared
region (CaF2, SiO2).

Silicon is one from possible IR
spectroscopy materials which is often used for ATR technique, but has not been
used for transmittance measurements so frequently yet. NICODOM with its over 10
years long experience in FTIR spectroscopy recommends you silicon as an
excellent infrared window material, because it is overcoming the main mentioned
disadvantages: it is not hygroscopic and is chemically very resistant, resists
thermal and mechanical shocks, scratching and fogging and is inexpensive.
However, silicon exhibits relatively high refractive index (3.4) and is not
transparent in visible region.

Material data

- Si is similar to Ge but it hase better resistance to thermal and
mechanical shock. Si is very inert, it is only soluble in hot HF or HF/HNO3
mixture. It reacts with F2.

The infrared spectrum of optical
grade silicon being used for NICODOM silicon windows shows that this material
is transparent in the standard middle infrared spectral region. The 100% line
of a NICODOM silicon window shows, that the silicon bands in the MIR region do
not exhibit total absorption and can be easily subtracted.